In a CDMA (Code Division Multiple Access) system, signals of multiple users are overlapped in time domain and frequency domain. Since signals of different users are correlated, signals of different users at the receiver will create some interference and this kind of interference is called MAI (multiple access interference). Although the MAI of one user is small, with an increase in user number or signal power, the MAI will become one of the most important interferences for the CDMA system.
The TD-SCDMA system may eliminate the above multiple access interference by the multi-user joint-detection technology to increase an anti-interference ability of the system. The multi-user joint-detection refers to that when the receiver separate the received signals, it doesn't regard the MAI as the interference signal but fully makes use of the prior information included in the MAI such as midambles used by each user and channelization codes used by each user, and separates signals of all the users from the received signals. Through the above multi-user joint-detection, the anti-interference ability of the TDD-CDMA system can be greatly improved and the capacity and coverage of the system is improved.
One premise to perform the multi-user joint-detection is to obtain user information of each user which specifically includes midambles and channelization codes used by each user. It is known that in a TD-SCDMA (Time Division Synchronous Code Division Multiple Access) system, a base station may perform a channel estimation by firstly judging which code channels are activated according to an activation detecting algorithm and then obtaining the midamble corresponding to each code channel according to a corresponding relation between each channelization code and each midamble so that it can further finish the multi-user joint-detection.
Generally, the corresponding relation between each channelization code and each midamble can be determined by a pre-defined allocation mode of the midambles or can be notified to the base station by the network side through a high layer signaling. According to the specification of protocols, there are three allocation modes for midambles which are default, common and specific. According to the TD-SCDMA protocol, the corresponding relation between each channelization code and each midamble is defined under different midamble allocation modes and at different channel estimation windows K.
Since there are many different allocation modes of midambles and in different allocation modes, the corresponding relation between each channelization code and each midamble is different and since allocations of midambles between neighboring cells are independent, no matter the base station or the UE (user equipment) can only obtain information of users in the present cell and cannot obtain information of users in a neighboring cell. Therefore, the above multi-user joint-detection is limited in the present cell and a uniform multi-user joint-detection cannot be performed on uplink signals of users in the present cell and the neighboring cell, i.e. the above multi-user joint-detection can only suppress the MAI between users in the present cell and cannot eliminate the MAI from users in a neighboring cell. In a case when the present cell works in different frequency than the neighboring cell, the MAI between users in the neighboring cell will not influence much on the system capacity and performance. However, in the case of co-frequency networking, the MAI between users in the neighboring cell cannot be neglected.